1. Field of the Invention
The present invention relates to a recording method with plural gradation levels for a magnetic printing machine and apparatus in order to get a smooth gradation without using a high frequency clock pulse.
2. Description of the Related Art
The present inventor have suggested a recording method with plural gradation levels using modulation of a recording frequency (see U.S. Pat. No. 4,163,979).
In the suggested method, an image is recorded on a recording medium using a recording pulse train with a low frequency for a picture signal corresponding to a black pixel and increasing frequency proportional to the increase of brightness of a pixel.
Not only analog modulation, but also digital modulation can be used for a frequency modulating circuit. When using an analog circuit, a continuous gradation is obtained but poor stability may be caused by temperature change and/or electronic element deterioration. To solve the above problem, the present inventor have suggested a magnetic printing machine applying digital modulation with good stability.
FIG. 7 shows one example of a digital magnetic printing machine. FIG. 7(a) shows a view of a recording drum 10 and a recording head 20, and FIG. 7(b) shows the development of the recording drum along a baseline 15--15'.
The recording drum 10 is driven in a direction shown by an arrow 11 by a drum motor 13 and a belt 12.
A rotary encoder 14 is directly connected to a shaft of the recording drum 10, and used for detection of a position of the recording head 20 from the baseline 15--15' and a rotating speed of the recording drum 10.
The recording head 20 is mounted on a carriage 24 driven by a head motor 22 and wire 23, and moved in a direction shown by an arrow 21.
A head position along the baseline 15--15' is detected by counting a number of drive pulses when a pulse-motor is applied as the head motor 22.
While the recording head 20 records a picture signal corresponding to one column along a circumference of the recording drum 10 on the recording drum 10, it is moved along the shaft of the recording drum 10, and records along a neighbouring circumference. By repeating the above operation, an image on one page is completed.
FIG. 8 shows a diagram of a frequency modulating circuit for the digital magnetic printing machine suggested by the present inventor.
A clock-pulse generator 31 generates clock-pulses synchronized with an output of the rotary encoder 14. These clock-pulses are applied to a decrement input terminal of a recording-pulse counter 37, and when every clock-pulse is input, a count value in the recording-pulse counter 37 is decremented.
An initial value for the recording-pulse counter 37 is set from a read only memory (ROM) 35 as follows.
For example, 4 bit picture code, that is, VIDEO 0-VIDEO 3 representing an original pixel in an original picture is applied to address terminals of ROM 35.
Memoried value corresponding to the address which is designated by the 4 bit picture code is set as the initial value for the recording-pulse counter 37.
An output signal of the recording-pulse counter 37 passes through a delay circuit 39, and is applied to an inverter 41 and a load terminal of the recording-pulse counter itself 37.
A count value of the recording-pulse counter 37 is decremented from the initial value which is determined by 4 bit picture code to zero when a clock-pulse is input, and the output of the recording-pulse counter 37 becomes zero when the count value reaches zero.
When the output of the recording-pulse counter 37 becomes zero, the initial value for the recording-pulse counter 37 is again read out from the ROM 35, because its output is fedback to its load terminal through the delay circuit 39.
Therefore, the output signal of the recording-pulse counter 37 keeps "1" logical level during the time which it is determined by (the initial value) x (the period of the recording clock-pulse), and turns to "0" logical level for a moment.
This output signal is applied to a flip-flop 43 through the inverter 41, and a pulse train having a period which is determined by (the initial value of the recording-pulse counter 37).times.(the period of the recording clock-pulse).times.2 and 50% duty ratio is got as the output of the flip-flop 43.
This pulse train is applied to a coil 26 of the recording head 20 after being amplified by two driver 45 and 47. Then an image is recorded on the recording drum 10 with a plural gradation level corresponding to a 4 bit picture signal.
In this case, 2.sup.4 =16 gradation levels are obtained, because a 4 bit picture signal code is used.
Table 1 shows a picture signal code, initial values for the recording-pulse counter memoried in ROM 37, reset intervals when a period of the clock-pulse is determined as 100 ns, a period of the output pulse of the flip-flop 43 and reflection density of recorded pixels.
For example, if the brightness of one original pixel in an original picture is gray, and the picture signal code corresponding to the pixel "8", the output of ROM 35 becomes "10". Then the recording-pulse counter 37 is reset every 10.times.100 ns=1000 ns=1 .mu.s. And pulse train having 2 .mu.s period and 50% duty ratio is output from the flip-flop 43.
The reflection density of the recorded pixel is given as 0.74.
FIG. 2 shows so-called 7-characteristic of the recorded pixels, and designates the .gamma.-characteristic when using the conventional recording method. In this graph, the abscissa designates the gradation level of original pixels in the original print, and the ordinate designates the reflection density of the recorded pixels.
This graph shows that the recorded pixels have only 9 gradation levels even though 16 gradation levels are used for recording.
If the digital modulation is applied for the magnetic printing machine, it is difficult to get the same levels of gradation as levels of digitized frequencies because of the following reasons.
(1) A magnetic drum used as a recording medium has a nonlinear magnetization characteristic.
(2) As recording frequencies changes discretely, gradation levels change stepwise.
Though smooth gradation can be obtained by using a high frequency clock and increasing the number of selectable recording frequencies, expensive circuit elements are required and poor stability results due to the influence of high-frequency noise.